used to had that too.
i just got my ip4, reloaded rm 10 then suddenly not long during later that day it ask me to top up to continue service.
the a pop-up window msg kept asking me to reload and was getting really annoyed.
forgot how was the msg my ip4 kept showing thou.
i guess my phone went to the internet or whether some app on my phone wanted to connect into the internet i guess.
still not sure of what happen.
i read from somewhere also that first time activation of facetime also that iphone would send a hidden international sms to the apple server in us and the user got charged( source from hardwarezone forum iphone)

had enough of this and port to digi unlimited plan to prevent any hidden charges incase it tries to connect to the internet again.
so far nothing happen when my bill came recently